The present invention generally relates to a liquid dispenser and method for dispensing fluids and more specifically, to an adjustable die for changing fluid flow through the fluid dispenser.
Various fluid dispensers have been developed for the precise placement of fluids. Many fluid dispensers have a valve stem with a valve body or ball on its distal end that is disposed on an upstream side of a valve seat. In operation, the valve ball is moved in an upstream direction to open the valve and in a downstream direction to close the valve. For purposes of this document, the term xe2x80x9cupstreamxe2x80x9d refers to a direction that is toward or closer to a source of fluid for the dispenser; and xe2x80x9cdownstreamxe2x80x9d refers to a direction that is further from the source of fluid. With the valve construction described above, when the valve opens, the valve ball is moving in an upstream direction against the direction of fluid flow and has a tendency to delay and disrupt the flow of fluid out of the dispensing nozzle. Similarly, when the valve closes, the valve ball is moving in the downstream direction with the direction of fluid flow; and the upstream valve ball has a tendency to cause a small additional quantity of fluid to be dispensed.
Many fluid dispensing applications require that the fluid be applied to the substrate with sharply defined boundaries, that is, the leading and trailing edges of the applied pattern of fluid on the substrate must be sharply defined or delimited. Thus, it is necessary that the motion of the valve ball be very fast, and the fluid flow be abruptly started and stopped to obtain sharp, square, cut-on and cut-off edges with no stringing of material. Thus, the tendency of the upstream valve ball to dispense even small quantities of fluid after the gun and valve have been shutoff is undesirable.
In order to provide a sharper initiation and cut-off of fluid flow, it is known to snuff-back, that is, temporarily buffer or capture any stringing material or other post shut-off fluid flow before it is deposited on the substrate. The buffered or captured fluid is then deposited during a subsequent fluid dispensing cycle. There are several known approaches to achieve the desired snuff-back function. In one application, excess material is removed from, or held within, a discharge slot of a nozzle by a negative pneumatic pressure created therein immediately upon the dispensing valve being shutoff. Thus, the excess material is not discharged from a nozzle of the fluid dispenser. That material is then discharged from the fluid dispenser during a successive dispensing cycle. Such systems effectively minimize post valve-shutoff fluid dispensing. However, such negative pressure pneumatic systems have extensive additional components; and, in some designs, cylinder chambers are provide in order to buffer the post valve-shutoff fluid. Consequently, such negative pressure designs add significant cost to the dispensing fluid system.
It is also known to provide a sharper initiation and cut-off of fluid flow using a snuff-back valve construction. With this construction, the valve ball is disposed on a downstream side of a valve seat. The valve ball is moved in the downstream direction away from the valve seat to open the valve and in the upstream direction toward the valve seat to close the valve. Consequently, as the valve opens, the valve ball is moving in the same downstream direction as the viscous fluid; and the viscous fluid begins to be dispensed simultaneously with the opening of the valve ball. When the valve closes, the valve ball is moving in the upstream direction and is effective to sharply cut-off the flow of viscous fluid. While such snuff-back valves operate effectively to a provide sharper cut-on and cut-off of fluid flow, snuff-back valves have a more expensive and complicated construction than valves having an upstream valve ball.
Even though such known snuff-back devices work reasonably well, there is a continuing effort to improve the performance and/or reduce the cost of implementing a snuff-back feature. Thus, there is a need for a snuff-back capability that eliminates the complexities of a negative pressure system and can be utilized with known, lower cost, upstream ball valve designs.
The present invention provides a fluid dispensing die that substantially improves the quality of a fluid dispensing operation. The fluid dispensing die of the present invention is capable of changing its physical geometry and die slot volume to provide increased control over fluid flow during the fluid dispensing process. Further, the die slot volume can be changed in real time to provide a greater volume upon a fluid dispensing valve being shut off, thereby obtaining a snuff-back capability. The fluid dispensing die of the present invention is especially useful with slot die applications where a sharp cut-off of fluid flow is required. The present invention provides a slot die that can be used with dispensing valves that have the valve ball upstream of the valve seat. Such valves are simple, reliable and less expensive than valves designed to provide a snuff-back capability. Hence, the use of dispensing valves having upstream valve balls in slot die applications presents an opportunity for substantial savings.
According to the principles of the present invention and in accordance with the described embodiments, the invention provides a die for dispensing a fluid onto a substrate. The die has first and second lips that define a die cavity therebetween. An actuator is connected to the first lip and is operable to move the first lip relative to the second lip, thereby changing a size of the die cavity.
In one aspect of the invention, the die is a slot die and the actuator can be an electromechanical actuator, a fluid actuator or an electrohydraulic actuator.
In another embodiment of the invention, a fluid dispensing apparatus is provided for dispensing fluid from a fluid source onto a substrate. The fluid dispenser has a fluid dispensing valve connected to the fluid source and a die connected to the fluid dispensing valve. The die has first and second lips that define a die slot therebetween. A lip actuator is connected to the first lip and is operable to move the first lip with respect to the second lip, thereby changing a volume of the die slot between the first and second lips.
In one aspect of this invention, the fluid dispensing valve has a valve ball upstream of the valve seat. Further, the lip actuator moves the first lip relative to the second lip to increase a volume of the die slot so that extra fluid dispensed by the upstream ball valve is maintained in the increased volume of the die slot.
In another embodiment of the invention, a method is provided for automatically changing a size of a die cavity through which fluid is dispensed onto a substrate. A first die lip is positioned adjacent a second die lip to form the die cavity therebetweeen. The first die lip is automatically moved with respect to the second die lip with an actuator in association with a fluid dispensing cycle, thereby changing the volume of the die cavity.
In one aspect of this invention, the first die lip is moved away from the second die lip in association with a closing of a fluid dispensing valve, thereby increasing the volume of the die cavity, so that extra fluid dispensed by the fluid dispensing valve in the process of closing the fluid dispensing valve is contained in the die cavity. In another aspect of this invention, the first die lip is moved toward the second die lip in association with an opening of a fluid dispensing valve, thereby decreasing the volume of the die cavity, so that the extra fluid in the die opening is dispensed onto the substrate.
These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein